I've been flying every week from SFO to CHI for work and have been using United for the most part for its Star Alliance network and since I'm currently silver status (which isn't much). However, I'm also an Southwest a-list member and have been booking flights when UA is outrageously more expensive on a given flight (15% of my travel this year). Question to veteran road warriors - is travelling two airlines something you guys/gals typically do for flexibility, cost, or other reasons? I'm not paying for my flights but when I see something that's half the cost, I can't help it but to go with the lower cost carrier. The pros I see is that I can gain mid tier statuses on both airlines but the con is that I'd never make it to 1K on United.

As a side question, how does UA get away with almost charging double over AA, Virgin, and Southwest for SFO - ORD during similar flight schedules?